The present invention relates to an optical system for an observing device such as a binocular or a terrestrial telescope that has an image-vibration compensation system.
Recently, binoculars provided with image-vibration compensation systems for preventing vibration of an observed image due to a hand-held shake of a user have been developed. Japanese Laid Open Publication No. HEI 6-43365 discloses an image-vibration compensation system of a binocular that employs a variable angle prism for each of the telescopic optical systems of the binocular. FIG. 37 shows one example of the telescopic optical system that has the same arrangement of the optical components as the publication. The optical system includes an objective lens 1 that is a cemented doublet, a variable angle prism 2 as a compensation element, an erecting system 3 and an eyepiece 4. When the optical system vibrates due to hand-held shake of a user, the vertex angle of the variable angle prism 2 is controlled to stabilize the image.
In such a construction, however, since the variable angle prism 2 is located in the convergent light, if the vertex angle of the prism is changed, significant decentering coma occurs. FIG. 38 shows the axial coma at a standard condition where the vertex angle of the variable angle prism 2 is 0 degrees. FIG. 39 shows the axial coma when the variable angle prism 2 is angled to stabilize the image when the optical system is inclined by 1 degree. The image viewed by a user is deteriorated due to the decentering coma when the variable angle prism 2 is angled to compensate for the vibration of the image. While FIGS. 38 and 39 show the change of the coma in the specific example, the image vibration compensation system having the variable angle prism tends to generate the decentering coma in general.
Another example of the image-vibration compensation system for binoculars is disclosed in Japanese Laid Open Publication No. HEI 6-308431. The binocular in the publication employs a compensation device that is attached in front of the objective optical systems of the binocular. This arrangement can reduce the decentering coma when the compensation prism is angled. However, since the first lens of the objective optical system has the largest diameter in the telescopic optical system, in general, and the device is arranged in front of the first lenses, the device becomes large in size, which increases the size of the binocular.